Clamping apparatus for cleaving a bonded wafer structure

CROSS-REFERENCE TO RELATED APPLICATION

0001The present application claims the benefit of U.S. Provisional Application No. 61/553,447 filed Oct. 31, 2011, which is incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

0002This disclosure generally relates to apparatus and methods for cleaving a bonded wafer structure and, more particularly, to apparatus and methods that involve clamps that are actuated to cause the bonded structure to cleave.

BACKGROUND

0003Semiconductor wafers are generally prepared from a single crystal ingot (e.g., a silicon ingot) which is sliced into wafers. The wafers are ground and/or polished to produce product wafers. While reference will be made herein to semiconductor wafers constructed from silicon, other materials may be used as well, such as germanium or gallium arsenide.

0004One type of wafer is a silicon-on-insulator (SOI) wafer. An SOI wafer includes a thin layer of silicon atop an insulating layer (e.g., an oxide layer), which is in turn disposed on a silicon substrate.

0005An exemplary process of making an SOI wafer includes depositing a layer of oxide on a polished front surface of a donor wafer and/or a handle wafer. Particles (e.g., hydrogen atoms or a combination of hydrogen and helium atoms) are implanted at a specified depth beneath the front surface of the donor wafer. The implanted particles form a cleave plane in the donor wafer at the specified depth at which they were implanted. The surface of the donor wafer is cleaned to remove organic compounds deposited on the wafer during the implantation process.

0006The front surface of the donor wafer is then bonded to a handle wafer to form a bonded wafer pair through a hydrophilic bonding process. The donor wafer and handle wafer are bonded together by exposing the surfaces of the wafers to plasma containing, for example, oxygen or nitrogen. Exposure to the plasma modifies the structure of the surfaces in a process often referred to as surface activation. The wafers are then pressed together and a bond is formed therebetween. This bond is relatively weak, and must be strengthened before further processing can occur.

0007In some processes, the hydrophilic bond between the donor wafer and handle wafer (i.e., a bonded wafer) is strengthened by heating or annealing the bonded wafer pair at temperatures between approximately 300° C. and 500° C. The elevated temperatures cause the formation of covalent bonds between the adjoining surfaces of the donor wafer and the handle wafer, thus solidifying the bond between the donor wafer and the handle wafer. Concurrently with the heating or annealing of the bonded wafer, the particles earlier implanted in the donor wafer weaken the cleave plane. A portion of the donor wafer is then separated (i.e., cleaved) along the cleave plane from the bonded wafer pair to form the SOI wafer.

0008The bonded wafer is first placed in a fixture in which mechanical force is applied perpendicular to the opposing sides of the bonded wafer in order to pull a portion of the donor wafer apart from the bonded wafer. According to some methods, suction cups are utilized to apply the mechanical force. The separation of the portion of the donor wafer is initiated by applying a mechanical wedge at the edge of the bonded wafer at the interface between the donor wafer and the handle wafer. The application of the mechanical force initiates propagation of a cleave along the cleave plane. The mechanical force applied by the suction cups then pulls a portion of the donor wafer away from the bonded wafer, thus forming an SOI wafer.

0009The resulting SOI wafer thus comprises a thin layer of silicon disposed atop the oxide layer and the handle wafer. The thickness of the layer may be non-uniform. The layer may also have a non-uniform roughness. This non-uniform thickness and roughness of the layer may be the result of the cleave propagating at varying speeds and/or the mechanical force applied by the suction cups. Additional processing is thus required to reduce the variation in thickness of the layer and/or smooth this layer. These additional processing steps are both time consuming and costly.

0010Thus, there remains a continuing need for a system and method for cleaving a bonded wafer structure that results in the structure having a layer with a relatively uniform thickness and roughness.

0011This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

SUMMARY

0012One aspect of the present disclosure is directed to an apparatus for cleaving a bonded wafer structure that includes a first wafer and a second wafer. The first wafer has a peripheral edge that includes a first portion and the second wafer has a peripheral edge that includes a second portion. The first portion is disposed opposite the second portion. The apparatus has a first arm, a first clamp, a second arm and a second clamp. The first clamp is mounted on the first arm and has a surface for contacting the first portion of the peripheral edge of the first wafer. The second clamp is mounted on the second arm and has a surface for contacting the second portion of the peripheral edge of the second wafer.

0013Another aspect of the present disclosure is directed to a method of cleaving a bonded wafer structure that includes a first wafer and a second wafer. The first wafer has a peripheral edge that includes a first portion and the second wafer has a peripheral edge that includes a second portion. The first portion is disposed opposite the second portion. The first portion of the peripheral edge of the first wafer is contacted with a surface of a first clamp mounted on a first arm. The second portion of the peripheral edge of the second wafer is contacted with a surface of a second clamp mounted on a second arm. The bonded wafer structure is cleaved by at least one of (1) applying an upward force on the first wafer by actuating the first clamp upward or (2) applying a downward force on the second wafer by actuating the second clamp downward.

0014A further aspect of the present disclosure is directed to an apparatus for cleaving a bonded wafer structure that includes a first wafer and a second wafer. The first wafer has a peripheral edge that includes a first portion and the second wafer has a peripheral edge that includes a second portion. The first portion is disposed opposite the second portion. The apparatus includes a first grasping member and a second grasping member. The first grasping member has one or more annular members. The annular members have a surface for contacting the first portion of the peripheral edge of the first wafer. The second grasping member has one or more annular members. The annular members have a surface for contacting the second portion of the peripheral edge of the second wafer.

0015Yet another aspect of the present disclosure is directed to a method of cleaving a bonded wafer structure that includes a first wafer and a second wafer. The first wafer has a peripheral edge that includes a first portion and the second wafer has a peripheral edge that includes a second portion. The first portion is disposed opposite the second portion. The first portion of the peripheral edge of the first wafer is contacted with a surface of an annular member of a first grasping member. The second portion of the peripheral edge of the second wafer is contacted with a surface of an annular member of a second grasping member. The bonded wafer structure is cleaved by at least one of (1) applying an upward force on the first wafer by actuating the first grasping member upward or (2) applying a downward force on the second wafer by actuating the second grasping member downward.

0016Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

0017<figref idref="DRAWINGS">FIG. 1</figref> is a top view of an apparatus for cleaving a bonded wafer structure;

0018<figref idref="DRAWINGS">FIG. 2</figref> is a perspective view of the apparatus of <figref idref="DRAWINGS">FIG. 1</figref>;

0019<figref idref="DRAWINGS">FIG. 3</figref> is a cross-sectional view of <figref idref="DRAWINGS">FIG. 1</figref> taken along the <b>3</b>-<b>3</b> line showing the apparatus prior to cleaving the bonded wafer structure;

0020<figref idref="DRAWINGS">FIG. 4</figref> is a cross-sectional view of <figref idref="DRAWINGS">FIG. 1</figref> taken along the <b>4</b>-<b>4</b> line showing the apparatus prior to cleaving the bonded wafer structure;

0021<figref idref="DRAWINGS">FIG. 5</figref> is an enlarged portion of <figref idref="DRAWINGS">FIG. 3</figref>;

0022<figref idref="DRAWINGS">FIG. 6</figref> is an enlarged portion of <figref idref="DRAWINGS">FIG. 4</figref>;

0023<figref idref="DRAWINGS">FIG. 7</figref> is a perspective view of a clamp used in an apparatus for cleaving a bonded wafer structure;

0024<figref idref="DRAWINGS">FIG. 8</figref> is a top view of an apparatus used for cleaving a bonded wafer structure before securing the structure;

0025<figref idref="DRAWINGS">FIG. 9</figref> is a top view of an apparatus used for cleaving a bonded wafer structure after securing the structure;

0026<figref idref="DRAWINGS">FIG. 10</figref> is a cross-sectional view of Figure taken along line <b>10</b>-<b>10</b>;

0027<figref idref="DRAWINGS">FIG. 11</figref> is a top view of an apparatus for cleaving a bonded wafer;

0028<figref idref="DRAWINGS">FIG. 12</figref> is black and white ACCUMAP® image of a surface of a SOI wafer after cleaving by a conventional process using suction cups; and

0029<figref idref="DRAWINGS">FIG. 13</figref> is black and white ACUMAP® image of a surface of a SOI wafer after cleaving by a process in which opposite forces are applied at the bonded wafer edge.

0030Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION

0031The embodiments described herein generally relate to apparatus and methods for mechanically cleaving a bonded wafer structure such as a bonded wafer pair. The apparatus and methods cleave (i.e., separate) the bonded structure along a cleave plane within the structure to form a new structure such as a silicon-on-insulator (SOI) wafer. While reference is made herein to use of the apparatus and methods to form silicon-on-insulator structures, the systems and methods can also be used to form other structures without limitation.

0032<figref idref="DRAWINGS">FIGS. 1-7</figref> depict an apparatus <b>100</b> for mechanically cleaving a bonded structure <b>102</b>. The bonded structure <b>102</b> shown in <figref idref="DRAWINGS">FIGS. 1-7</figref> is a bonded wafer pair. However, it should be noted that structures other than a bonded wafer pair may be cleaved by use of the apparatus <b>100</b>. Referring to <figref idref="DRAWINGS">FIGS. 3 and 4</figref>, the structure <b>102</b> is separated along a cleave plane <b>140</b> shown in phantom. The bonded structure <b>102</b> has an upper surface <b>106</b> (generally, a first surface) and an opposing lower surface <b>108</b> (generally, a second surface). The bonded structure contains a handle wafer <b>110</b> (generally, a first wafer) bonded to a donor wafer <b>112</b> (generally, a second wafer) along a bond interface <b>104</b>. The distance between the cleave plane <b>140</b> in the donor wafer <b>112</b> and the bond interface <b>104</b> is exaggerated for the sake of clarity. Furthermore, in other embodiments, the position of the handle wafer <b>110</b> and the donor wafer <b>112</b> may be reversed such that the handle wafer is disposed beneath the donor wafer.

0033As shown in <figref idref="DRAWINGS">FIGS. 5 and 6</figref>, the donor wafer <b>112</b> and handle wafer <b>110</b> are beveled at their peripheral edge. It should be noted that the peripheral edge of the wafers may have a variety of geometries (e.g., rounded, bulleted). As used herein, the term “beveled” refers to a geometry in which the edge is not substantially perpendicular to the front and back surface of the wafer and has one or more tapered edges and should not be considered in a limiting sense. Further, it should be noted that, as used herein a “tapered edge” refers merely to a surface at the wafer edge that is not parallel to the front and back surfaces but rather slopes upward or downward toward the portion of the edge that is most distant from the center of the wafer. Further, such “tapered edges” may be straight or arcuate or may contain both straight and arcuate portions.

0034As also shown in <figref idref="DRAWINGS">FIGS. 5 and 6</figref>, the donor wafer <b>112</b> and handle wafer <b>110</b> have rounded edges. The edge of the donor wafer <b>112</b> has a first portion <b>148</b> and the edge of the handle wafer <b>110</b> has a second portion <b>152</b>, the first portion being disposed opposite the second portion thereby forming a depression <b>144</b> between the two edges. Generally the first portion <b>148</b> and second portion <b>152</b> are tapered edges of the wafer, the edges taping upward or downward depending on the orientation of the bonded wafer pair. As shown in <figref idref="DRAWINGS">FIGS. 5 and 6</figref>, the donor wafer <b>112</b> has a tapered edge <b>146</b> (synonymously “upwardly tapered edge”) that slopes upward from the lower surface <b>108</b> of the bonded structure toward the portion (or “point” as with rounded edges) <b>154</b> of the edge that is most distant from the center of the wafer. The donor wafer <b>112</b> also includes an edge <b>148</b> (synonymously “downwardly tapered edge” or “first portion” as described above) that slopes downward from the interface <b>104</b> toward the most outward portion <b>154</b> of the edge. Similar to the donor wafer <b>112</b>, the handle wafer <b>110</b> includes an upwardly tapered edge <b>152</b> (or “second portion” as described above) that slopes toward the most outward portion <b>156</b> of the edge of the handle wafer and a downwardly tapered edge <b>150</b> that tapers to the most outward portion <b>156</b> of the edge of the handle wafer <b>110</b>. The downward sloping edge <b>148</b> of the donor wafer <b>112</b> and the upward sloping edge <b>152</b> of the handle wafer <b>110</b> form a depression <b>144</b> between the two edges.

0035As shown in <figref idref="DRAWINGS">FIGS. 1-4</figref>, the apparatus <b>100</b> includes a plurality of first clamps <b>170</b> (or “upper clamps”) that releasably secure the handle wafer <b>110</b> and a plurality of second clamps <b>184</b> that releasably secure the donor wafer <b>112</b>. The clamps <b>170</b>, <b>184</b> are mounted respectively to first arms <b>160</b> and second arms <b>166</b>. The first arms <b>160</b> extend from a first central disk <b>190</b> and the second arms <b>166</b> extend from a second central disk <b>192</b>. The first central disk <b>190</b> may contact the upper surface <b>106</b> of the bonded structure <b>102</b> to properly align the first clamps <b>170</b> in relation to the structure <b>102</b>. Similarly, the second central disk <b>192</b> may assist in aligning the second clamps <b>184</b>. The first clamps <b>170</b>, first arms <b>160</b> and first central disk <b>190</b> together form a first grasping member <b>120</b> (<figref idref="DRAWINGS">FIG. 1</figref>) and the second clamps <b>184</b>, second arms <b>184</b> and second central disk <b>192</b> together form a second grasping member <b>122</b> (<figref idref="DRAWINGS">FIG. 4</figref>).

0036As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the first grasping member <b>120</b> has four arms <b>160</b> and four clamps <b>170</b> and the second grasping member <b>122</b> also has four arms <b>166</b> and four clamps <b>184</b>. However, it should be noted that each member <b>120</b>, <b>122</b> of the apparatus <b>100</b> may have more or less arms and clamps without limitation. In some embodiments, the apparatus <b>100</b> does not have arms but rather annular members for gripping the respective wafers as shown in <figref idref="DRAWINGS">FIGS. 8-10</figref> and described below.

0037Each first clamp <b>170</b> has a surface <b>130</b> (<figref idref="DRAWINGS">FIGS. 3 and 5</figref>) which contacts the upwardly tapered edge <b>152</b> of the handle wafer <b>110</b>. Similarly, each second clamp <b>184</b> has a surface <b>130</b> (<figref idref="DRAWINGS">FIGS. 4 and 6</figref>) that contacts the downwardly tapered edge <b>148</b> of the donor wafer <b>112</b>. As shown in further detail in <figref idref="DRAWINGS">FIG. 7</figref>, the end portion of the first clamp <b>170</b> has a curved profile such that the surface <b>130</b> of the arm may enter the depression <b>144</b> and contact the upwardly tapered edge <b>152</b> of the handle wafer <b>110</b> (<figref idref="DRAWINGS">FIG. 5</figref>). In this regard, the term “curved” should not be considered limiting and refers to a profile in which the arm generally changes its angular direction relative to other portions of the arm and may include one or more discreet changes in direction (e.g., the arm may form one or more angles within its length).

0038It should be noted that while the first clamp <b>170</b> is shown with a curvature of 180°, in other embodiments such as embodiments wherein the arm does not extend from the center of the bonded structure, the clamp may have a curvature other than 180°. For example, the arm <b>160</b> may be perpendicular to the front and back surfaces of the bonded structure and the clamp <b>170</b> may curve about 90° into the depression <b>144</b> to contact its surface <b>130</b> with the tapered edge <b>152</b>. In some embodiments, the clamp <b>170</b> does not have a curved profile such as embodiments wherein the arm extends from the opposite direction as the arm shown in <figref idref="DRAWINGS">FIG. 7</figref> (i.e., from outside of the bonded structure). In this regard, the second clamp <b>184</b> may have a shape and/or dimensions similar to the first clamp <b>170</b>. However, the second clamp <b>184</b> is oriented opposite that of the first clamp <b>170</b> to allow the second clamp <b>184</b> to grasp the donor wafer <b>112</b> (i.e., the clamp <b>184</b> curves upward towards the depression <b>144</b> (<figref idref="DRAWINGS">FIG. 5</figref>) rather than downward). The clamps <b>170</b>, <b>184</b> may have other shapes and/or dimensions (e.g., different from each other) without limitation.

0039As shown in <figref idref="DRAWINGS">FIG. 7</figref>, the first clamp <b>170</b> converges to form an edge which is the surface <b>130</b> which contacts the handle wafer <b>110</b> of the bonded structure <b>102</b>. However, in other embodiments, the surface <b>130</b> may also have a width (i.e., the surface may be rectangular or square rather than an edge) or may be a singular point which contacts the surface. Further, the surface <b>130</b> may be arcuate and optionally may have a shape which is complementary to the tapered edge of the handle wafer to increase contact between the handle wafer and the surface <b>130</b>. The surface <b>130</b> of the second clamp <b>184</b> may have a shape described above in regard to the surface <b>130</b> of the first clamp <b>170</b>.

0040To separate the bonded structure <b>102</b>, the first grasping member <b>120</b> and/or the second grasping member <b>122</b> grasps that bonded structure. Before being grasped, the bonded structure <b>102</b> may be moved into position by robotic means or by use of one of more of the grasping members <b>120</b>, <b>122</b>. Before cleaving, the other grasping member grasps the structure. The bonded structure <b>102</b> may be grasped by the first grasping member <b>120</b> by extending and retracting one or more of the arms <b>160</b> of the member <b>120</b> by use of a first actuator (not shown) that forms part of or is attached to the first member <b>120</b>. Similarly, the bonded structure <b>102</b> is grasped by the second grasping member <b>122</b> by use of a second actuator (not shown) that is attached to or forms part of the second member <b>122</b> to extend and retract one or more arms <b>166</b> of the second grasping member <b>122</b>. Actuation of the arms <b>160</b>, <b>166</b> of the first grasping member <b>120</b> and second grasping member <b>122</b> may be achieved by any method known to those of skill in the art including, for example, pneumatic, hydraulic or mechanical actuation.

0041After the bonded structure <b>102</b> is grasped by the first grasping member <b>120</b> and the second grasping member <b>122</b>, a force is applied to one or both of the first grasping member <b>120</b> and second grasping member <b>122</b> to separate the bonded structure <b>102</b> at the cleave plane <b>140</b>. In this regard, an actuator (not shown) may be attached to the first grasping member <b>120</b>. The actuator causes the first grasping member <b>120</b> to apply an upward force to the handle wafer <b>110</b> to separate the structure. Alternatively or additionally, an actuator (the same one or a different actuator) may be attached to the second grasping member <b>122</b> that is operable to cause the second grasping member <b>122</b> to apply a downward force on the donor wafer <b>112</b>. The actuator(s) may be attached to the shafts <b>180</b>, <b>182</b> or may be attached to other portions of the grasping members <b>120</b> or <b>122</b>. It should be noted that only one of the first grasping member <b>120</b> and second grasping member <b>122</b> needs to be actuated to provide the force used to separate the bonded structure and the other clamp may be maintained in a stationary position. However, in some embodiments both the first grasping member <b>120</b> and second grasping member <b>122</b> are actuated.

0042The cleaving process may be initiated by use of a blade (not shown) that exerts force on the bonded structure <b>102</b>. The blade is inserting into the depression <b>144</b> formed between the handle wafer <b>110</b> and donor wafer <b>112</b> and exerts force along the bonded structure edge. The force results in the initiation of a cleave along the cleave plane <b>140</b>. This cleave in combination with the force exerted by one or both of the grasping members <b>120</b>, <b>122</b> results in the cleaving and separation of the bonded structure along the cleave plane <b>140</b>. The blade is movable in a lateral direction by an actuator (not shown) or other suitable mechanism.

0043In some embodiments of the present disclosure and as shown in <figref idref="DRAWINGS">FIGS. 8-10</figref>, rather than having a plurality of arms, the grasping member may include one or more annular members which at least partially encompass the bonded structure. As shown in <figref idref="DRAWINGS">FIG. 8</figref>, the first grasping member <b>120</b> includes a first annular member <b>194</b> and a second annular member <b>196</b> to grasp the handle wafer <b>110</b> of the bonded structure. The second grasping member <b>122</b> (<figref idref="DRAWINGS">FIG. 10</figref>) may also have a first annular member <b>172</b> and a second annular member <b>188</b> for grasping the donor wafer <b>112</b>.

0044The first annular member <b>194</b> and second annular member <b>196</b> of the first grasping member <b>120</b> each have a surface <b>130</b> (<figref idref="DRAWINGS">FIG. 10</figref>) that contacts the first upwardly tapered edge of the handle wafer <b>110</b>. Similarly, the first annular member <b>172</b> and second annular member <b>188</b> has a surface <b>130</b> that contacts the downwardly tapered edge of the donor wafer <b>112</b>. As indicated by arrows in <figref idref="DRAWINGS">FIG. 8</figref>, the annular members <b>194</b>, <b>196</b> are actuated to contact the handle wafer <b>110</b> at its edge (<figref idref="DRAWINGS">FIG. 9</figref>). The annular members <b>172</b>, <b>188</b> of the second grasping member <b>122</b> may be actuated in a similar manner. Once the handle wafer and donor wafer are grasped, the first grasping member <b>120</b> and/or second grasping member <b>122</b> may be actuated to apply opposing forces and cause the bonded structure to cleave.

0045It should be noted that the annular members of the first grasping member <b>120</b> and second grasping member <b>122</b> need not be arranged such that the handle wafer <b>110</b> and donor wafer <b>112</b> are contacted continuously at their respective peripheral edges. The annular members may contact discreet portions of the peripheral edge (i.e., the annular members may have a length less than half the circumference of the bonded structure when the grasping member has two annular members). Further, each grasping member may have more than two grasping members without limitation.

0046In some embodiments, one or both of the grasping members are composed of one annular member (i.e., the grasping member is a continuous annulus) that has an actuated edge that allows the grasping member to change diameter to allow the donor and/or handle wafer to be grasped and released after cleaving. An exemplary grasping member <b>120</b> is shown in <figref idref="DRAWINGS">FIG. 11</figref>. The grasping member <b>120</b> includes an actuated lip <b>176</b> which may have one or more actuated portions <b>108</b> which change the inner diameter D of the grasping member upon actuation. The grasping member moves axially with respect to the bonded structure to grasp the bonded structure (i.e., approaches the bonded structure from above or below the structure) rather than moving radially as with the grasping members <b>120</b>, <b>122</b> shown in <figref idref="DRAWINGS">FIGS. 8-10</figref>.

0047Without being held to any particular theory, it is believed that non-uniform thickness and/or roughness variations in the layers of structures (e.g., SOI structures) produced according to previous systems were caused by localized stress applied to the surface of the bonded structure. The embodiments described herein reduce or eliminate this problem by applying force at the wafer edge thereby reducing or eliminating thickness and/or roughness variations in the resulting device layer.

EXAMPLES

0048The processes of the present disclosure are further illustrated by the following Examples. These Examples should not be viewed in a limiting sense.

Example 1

Comparison of the ACCUMAP® Image of a SOI Structure Cleaved by Use of Suction Cups and a SOI Structure Cleaved by Applying Opposing Forces at the Wafer Edge

0049A bonded wafer pair was cleaved by a conventional process that used suction cups to separate the donor wafer from the resulting SOI structure. The cleave was initiated by used of cleave blade. A second SOI structure was also prepared by applying an upward and downward force to the bonded structure after the cleave was initiated by the cleave blade.

0050After the wafer was cleaved to produce the SOI structures, the surface of the device layer was imaged to produce an ACCUMAP® image (ACCUMAP® 3220; ADE). The image produced from the SOI structure cleaved by the conventional method that used suction cups is shown in <figref idref="DRAWINGS">FIG. 12</figref> and the image produced from the SOI structure cleaved by applying opposite forces at the bonded wafer edge is shown in <figref idref="DRAWINGS">FIG. 13</figref>. As can be seen from <figref idref="DRAWINGS">FIG. 12</figref>, the SOI structure produced by using suction cups during the cleave had strong cleave striations at the bottom, far left and far right of the image. The SOI structure of <figref idref="DRAWINGS">FIG. 13</figref> is more symmetrical and results in greater thickness uniformity.

0051When introducing elements of the present disclosure or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” “containing” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described.

0052As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.